We will measure imunoreactive plasma leptin concentrations in 400 subjects under basal and experimental conditions and stratify results by body mass index (BMI) into four strata. We will then measure insulin sensitivity by clamp method in 64 subjects comprising individuals who are matched for age, gender, and BMI, but discordant for leptin secretor status {"low secretors"(plasma leptin<10ng.dl) and "high secretors" (>20 ng/dl}.